in the new NFS server for NFSv4, where it would report ENOENT
when the file actually existed on the server. This turned out
to be caused by not initializing ni_topdir before calling lookup()
and there was a rare case where the value on the stack location
assigned to ni_topdir happened to be a pointer to a ".." entry,
such that "dp == ndp->ni_topdir" succeeded in lookup().
This patch initializes ni_topdir to fix the problem.
MFC after: 5 days